On Wed, 10 Jun 2015, David Luposchainsky wrote:
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On 10.06.2015 00:26, Johan Tibell wrote:
"As a consequence, in current Haskell, you can not use Monad-polymorphic code safely, because although it claims to work for all Monads, it might just crash on you. This kind of implicit non-totality baked into the class is terrible."
Is this actually a problem in practice? Is there any code we can point to that suffers because of the current state of affairs? Could it be included in the proposal?
I don't have hard evidence, but the Monad class being partial strikes me as pretty backwards in a language where totality and no implicit failures are important to the programmers. We try our best to advocate not using certain functions like "head" carelessly, but do-notation comes with similar partiality.
As far as I remember, I have never implemented 'fail' in the Monad instances I wrote over the years. Thus, splitting off MonadFail would reward me with more precise type signatures: The constraint (Monad m) would prevent people from calling 'fail' which I have never implemented.